Add openstack-infra/askbot-theme.

Change-Id: I6940e49deaa2889e30327c202272176477f82593
Reviewed-on: https://review.openstack.org/25021
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins